Heretofore, various aromatic dicarboxylic acids have been known and widely used as raw materials for condensation polymers such as polyesters and polyamides. For example, terephthalic acid is industrially used as a raw material for polyethylene terephthalate, polytetraethylene terephthalate and polyparaphenylene terephthalamide. Isophthalic acid is also widely used as a raw material for polymers. It is known that polyethylene naphthalene dicarboxylate (polyethylene naphthalate) which is a polyester obtained from naphthalenedicarboxylic acid is superior to polyethylene terephthalate in mechanical properties and heat resistance.
Further, demand for raw materials having excellent physical and chemical properties is growing due to progress in technology. For instance, polyethylene terephthalate is widely used in resins, fibers and films, and polyethylene terephthalate films are widely used as they have excellent heat resistance, strength and Young's modulus. However, it is still hard to say that they have satisfactory properties depending on application purpose. Although a liquid crystalline polyester is an excellent material having a high Young's modulus, high melting point and high fluidity, when it is melt molded, it is readily oriented in a monoaxial direction and therefore it is difficult to obtain a biaxially oriented film like polyethylene terephthalate.
As a polyester having higher performance than polyethylene naphthalate, there is proposed an aromatic polyester comprising 6,6′-(alkylenedioxy)di-2-naphthoic acid (may be referred to as “NEO-H” hereinafter) as a dicarboxylic acid component. NEO-H has low solubility in a diol component such as ethylene glycol when it is in an acid form and it is difficult to handle it at the time of a polymerization reaction. Therefore, it is proposed to manufacture an aromatic polyester by transesterifying an ethyl ester of NEO-H (may be referred to as “NEO-ester” hereinafter) with ethylene glycol and polycondensing the resulting product (patent documents 1 to 4).
However, since the transesterification reaction in this case is carried out while ethanol formed by the reaction is discharged, the amount of ethylene glycol decreases along with the proceeding of the reaction, and a polyester precursor formed by the reaction is crystallized to be solidified, whereby stirring becomes difficult and the reaction time becomes long. The obtained aromatic polyester is apt to contain a diethylene glycol component represented by the formula (D) which tends to deteriorate the physical properties of the aromatic polyester.
Since the NEO-ester has an extremely high boiling point, it is difficult to distill it, and purification by recrystallization must be carried out, thereby making it difficult to purify it at a low cost. Consequently, it is difficult to manufacture an aromatic polyester on an industrial scale from the NEO-ester as a starting material.    (Patent Document 1) JP-A 60-135428    (Patent Document 2) JP-A 60-221420    (Patent Document 3) JP-A 61-145724    (Patent Document 4) JP-A 6-145323